Typical three-phase reluctance type motor comprises a stator having six magnetic poles and a rotor having four or eight salient-poles. Three sets of magnetic poles, each set consisting of two magnetic poles facing each other in a diameter direction of the motor, are associated with three sets of exciting coils constituting a first to a third phase exciting coils, respectively. When exciting current is successively supplied to the first to the third exciting coils, the magnetic poles are excited to cause magnetic attraction force between the excited magnetic poles and their corresponding salient-poles. And, this magnetic attraction force causes the rotor to rotate.
In a reluctance type motor, even when a magnetic flux penetrating magnetic poles is in a saturated condition, an output torque of the motor increases in accordance with increase of a magnetomotive force (i.e. an ampere-turn) defined by a number of turns of an exciting coil multiplied by its exciting current. For example, in case of a reluctance motor of approximately 500 watt output, as shown in FIG. 1, an output torque increases in proportion to a square value of an exciting current in a region 3a in which the exciting current is smaller than, for example, 2 ampere, and to the contrary it increases in proportion to the exciting current itself in a region 3b in which the exciting current is larger than 2 ampere.
Accordingly, theoretically, the output torque of motor can be increased by increasing magnetomotive force of exciting coils. However, an available space inside of the motor which the exciting coils can occupy is limited, therefore it is difficult to scale up the exciting coil in order to increase number of turns of the exciting coil. Furthermore, an increase of the exciting current is accompanied with increase of copper loss. Thus, it is difficult to increase an output torque of the reluctance type motor by increasing magnetomotive force of the exciting coil.
Generally, in a three-phase reluctance type motor, an initiation and a termination of exciting current supply coincides with a 180-degree electric angle rotation of a rotor. That is, during one complete revolution of the rotor, an accumulation and an extinction of magnetic flux are repeated 6 times. Such a large number of repetition of the accumulation and the extinction of magnetic flux during one complete revolution of the rotor increases iron loss in the reluctance type motor. Moreover, since an inductance of the exciting coil is large, magnetic energy stored in the exciting coil becomes remarkably large. Therefore, it requires significant time for completion of accumulation and extinction of magnetic energy. For this reason, a building-up and a trailing edge of the exciting current are delayed undesirably.
Accordingly, not only torque reductions occur (i.e. the torque is reduced), but also counter torques are generated. However, since these torque reduction and counter torque generation are increased in accordance with increase of rotational speed of the motor, it is difficult to cause the motor to rotate in a high speed region.
Especially, if the number of salient-poles and magnetic poles is increased in order to increase an output torque of the motor, or a gap between the salient-pole and the magnetic pole is set to be small, a period of time required for the building-up or the trailing edge of the exciting current derived from stored magnetic energy further increases. Thus, the rotational speed remarkably decreases.
Consequently, in a conventional reluctance type motor, it was difficult to realize both a required rotational speed and a large output torque.